The present invention generally relates to delivery or transfer systems for cryogenic liquids and, more particularly, to a transfer system that delivers liquified natural gas (LNG) fuel to a vehicle fuel tank without using a pump or compressor and conditions the LNG to the desired temperature and pressure while keeping the pressure in the system's bulk storage tank at a desired low level.
LNG is one alternative energy source which is domestically available, environmentally safe and plentiful when compared to oil. As a result, the use of LNG as a fuel for vehicles such as buses, trucks and the like has greatly increased. Entire fleets of government and industry vehicles, as well as some privately-owned vehicles, have been successfully converted to LNG power. These developments have necessitated a focus on the development of LNG transfer systems for delivering natural gas from a bulk storage tank to LNG-powered vehicles.
In contrast to conventional fuels such as gasoline, LNG is a cryogenic liquid and thus has a boiling point below -150.degree. F. at atmospheric pressure. Most LNG-powered vehicles, however, require that the LNG be delivered at a pressure above atmospheric pressure. This is because in the typical LNG-powered vehicle fuel system, the driving force to deliver the LNG from the vehicle fuel tank to the engine is the pressure of the fuel itself In other words, the vehicle employs no pump or other means of moving the fuel. Instead, the fuel is stored in the vehicle fuel tank at a pressure sufficient to force the fuel to the engine. It is thus necessary to increase the pressure of the LNG stored in the transfer system prior to its delivery to the vehicle.
Pressurizing the LNG stored in the transfer system by merely adding gas to the system storage tank, without heating the LNG stored therein, is ineffective. This is because the LNG, once delivered to the use vehicle, sloshes around in the use vehicle's fuel tank as the vehicle is driven. This results in condensation of the added gas which decreases the pressure of the LNG to a level that is below the requirements of the use vehicle. In order to avoid this condensation, the LNG must be in a saturated state at the higher pressure level. In other words, the pressurization must result in an equilibriated pressure.
This pressurization is accomplished by heating the LNG to a higher temperature before delivery to the vehicle. This heating results in an increase in the pressure of the LNG until it reaches equilibrium at the saturation pressure for the higher temperature. The higher temperature is chosen so that its saturation pressure is approximately equal to the pressure required by the vehicle. The LNG is thus conditioned so as to be at the proper pressure required by the vehicle to which the pressurized LNG may then be distributed.
An increase in pressure of the stored LNG, however, makes filling of the bulk storage tank from a low pressure transport difficult or impossible without first venting the vaporized cryogen to reduce the pressure in the bulk storage tank. This venting is undesirable in that once the bulk storage tank is refilled, the pressurization process must be repeated, which means that more LNG must be boiled off as vapor. This decreases the amount of LNG available for distribution and is potentially hazardous. A need thus exists for a transfer system that can condition the LNG to a high pressure for vehicle use while maintaining a desired low pressure in the bulk storage tank.
Accordingly, an object of the invention is to provide a transfer system that can condition the cryogen to the desired pressure and temperature while maintaining a desired low pressure in the bulk storage tank.
Existing transfer systems commonly use pumps or compressors to establish the flow of pressurized LNG from the transfer system bulk storage tanks to LNG-powered vehicles. In addition, some transfer systems also use pumps or compressors to circulate the LNG through heating circuits for pressurization purposes. Such specialized pumps or compressors feature moving parts which wear and thus require repair, replacement and maintenance. These costs are considerable. Furthermore, pumps or compressors add considerable cost to the production, and thus purchase price, of a transfer system. These repair, replacement, maintenance and initial costs are multiplied for transfer systems that use a number of pumps and compressors. It would thus be a significant advantage if a transfer system could function without pumps or compressors.
As such, another object of the invention is to provide a cryogenic transfer system that conditions and delivers the cryogen without the need of a pump or compressor.